UV curable conformal coating with moisture shadow cure

ABSTRACT

A conformal coating composition is made from a mixture of acrylamide functional polydimethylsiloxane, a methacrylate functional/dialkoxy polydiorganosiloxane, reactive diluent including isobornyl acrylate, cyclohexyl acrylate, and 2- ethylhexyl acrylate; a photoinitiator combination of 2-hydroxy- 2-methylphenylpropanone and 2-methyl-1-[4-(methylthio)phenyl]-2- morpholenopropanone, an amine including dimethylethanolamine and diisopropylethylamine, and an organic titantate. The composition has a viscosity of from 0.07 to 0.2 Pa.s at 25° C. and cures by exposure to ultraviolet radiation and to atmospheric moisture, i.e. a dual curing composition. This composition cures too fast by exposure to ultraviolet radiation in the presence of nitrogen resulting in a distorted surface and therefore for coating printed circuit boards it is recommended that it be cured in air so that the rate of cure is reduced and no surface distortions are formed.

This application is a division of application Ser. No. 07/118,086, filedNov. 6, 1987, now U.S. Pat. No. 4,824,875.

BACKGROUND

1. Field of the Invention

This invention relates to a coating composition which can cure byexposure to ultraviolet radiation and also has capabilities of curing byexposure to moisture at room temperature. The ability to cure at roomtemperature provides the coating composition with a characteristic whichallows it to be used for coating application where the possibility ofexposing all the area of a coated substrate with ultraviolet radiationare so unlikely, so impractical, that one may say it is impossible. Thiscure process which cures without the exposure to ultraviolet radiationcan be termed "shadow cure" or "dark cure" meaning cure which occurswithout being exposed to the ultraviolet radiation, i.e. cures in theshadow of the exposure or in the dark areas when the ultravioletradiation is directed on the substrate.

2. Background Information

Compositions which cure by exposure to ultraviolet radiation are wellknown in the art, although the number of such materials found incommercial usage today are still relatively small. An even smallernumber of such compositions are known to the art which both cure byexposure to ultraviolet radiation and also by another mechanism, such asby heat. Compositions which cure by two mechanisms are said to have dualcure.

Compositions which cure by exposure to ultraviolet radiation and byexposure to moisture i.e. a dual cure, are also known in the art asshown by Lien et al in U.S. Pat. No. 4,528,081, issued July 9, 1985.Lien et al describe a dual curing silicone composition which contains anacrylic functional dialkoxy or diaryloxy silyl group terminatedpolyorganosiloxane, an effective amount of a photosensitizer, and asilicone moisture curing catalyst These compositions of Lien et al canalso contain trimethylsilyl terminated silicone oil. Lien et al describetheir materials as curing to soft gel like materials or soft rubberymaterials. In their examples the cured materials are defined as a softrubbery material at 0% oil to a very soft sticky gel at 70% oil whichshowed a tendency to creep but would not flow.

SUMMARY OF THE INVENTION

The present invention is a composition which has dual cure properties,i.e.. ultraviolet radiation cure and moisture cure. These compositionsare designed to be used as conformal coatings and have uncuredproperties, curing characteristics, and cured properties which makesthem particularly useful to coat printed circuit boards or printed wireboards.

This invention relates to a composition comprising (A) at least 20weight percent based on the total weight of the composition of anacrylamide polydimethylsiloxane having acrylamide end groups bonded tosilicon atoms through Si--C bonds and on the average at least 20 to lessthan 300 dimethylsiloxane units per molecule, (B) at least 20 weightpercent based on the total weight of the composition of apolydiorganosiloxane having endgroups containing both alkoxy groups andmethacrylate groups where there are two alkoxy groups per endgroupbonded to silicon through an Si--O bond and one methacrylate groupbonded to silicon through an Si--C bond and having on the average lessthan 350 diorganosiloxane units per molecule, said diorganosiloxaneunits being selected from dimethylsiloxane units, diphenylsiloxaneunits, and methylphenylsiloxane units wherein at least 50 percent of theorganic groups of the diorganosiloxane units are methyl (C) at least 20weight percent based on the total weight of the composition of areactive diluent selected from the group consisting of isobornylacrylate, cyclohexyl acrylate, and 2-ethylhexyl acrylate, (D) from 0.5to 10 weight percent based on the total weight of the composition of aphotoinitiator combination in which there is at least 0.25 weightpercent of

CH₆ H₅ --(O)C(CH₃)₂ --OH and at least 0.25 weight percent of ##STR1##(E) from 0.1 to 2 weight percent based on the total weight of thecomposition of an amine selected from the group consisting ofdimethylethanolamine and diisopropylethylamine, and (F) from 0.05 to 1weight percent based on the total weight of the composition of anorganic titanate, where the composition has a viscosity at 25° C. in therange of 0.07 to 0.2 Pa.s.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The compositions of the present invention cure extremely fast andbecause they cure so fast, it is necessary to cure them in the presenceof air (oxygen) to slow the rate of cure down. As conformal coatings,the fast cure in an inert atmosphere causes the surface to distort, suchas become wrinkled. The distortion causes an unattractive surface butprobably more importantly, it can also cause unacceptable stresses to beexerted on the components of a printed circuit board. Being able todevelop compositions having a low viscosity, having the ability to cureby both UV radiation and moisture to films which have low temperatureflexibility, and having a UV cure so fast under a nitrogen gasatmosphere that the cure is done in air to slow the reaction down, wasunexpected.

The acrylamide polydimethylsiloxane (A) contains at least 20 siloxaneunits and less than 300 siloxane units. These siloxane units contain twoacrylamide functional groups on the terminal units where the acrylamidegroups are bonded to silicon atoms through Si-C bonds. The acrylamidefunctional polydimethylsiloxanes are known in the art from Varaprath inU.S. Pat. No. 4,608,270, issued Aug. 26, 1986, and is herebyincorporated by reference to show the polydimethylsiloxanes withacrylamide functionality and to show the methods of making suchpolymers. The polydimethylsiloxanes which are useful in the presentinvention are those which have from 20 to less than 300 siloxane unitsand two terminal siloxane units with acrylamide groups. The siloxaneunits include dimethylsiloxane units and siloxane units of the formula##STR2## in which Z is H or R, R is a monovalent hydrocarbon radical, Qand Q' are divalent hydrocarbon radicals, A is an acyl radical havingthe formula ##STR3## where B is H or CH₃ and a is 0 or 1. The monovalenthydrocarbon radicals R, can be alkyl radicals such as methyl, ethyl,propyl, butyl, hexyl, and octyl; cycloaliphatic radicals such ascyclohexyl aryl radicals such as phenyl benzyl, styryl, tolyl, andxenyl; and alkenyl radicals such as vinyl and allyl. Q and Q' aredivalent hydrocarbon radicals such as ethylene, propylene, isopropylene,butylene, isobutylene hexylene, octylene, and phenylene. Q is preferablyethylene and Q' is preferably propylene or isobutylene. When B ishydrogen, the acyl radical is acrylyl radical and when B is methyl, theacyl radical is methacrylyl radical. Polydimethylsiloxanes endblockedwith ##STR4## are preferred when there are from 20 to 125 siloxane unitsper molecule. The most preferred acrylamide polydimethylsiloxanes of (A)are those which have from 70 to 110 siloxane units per molecule. Theamount of (A) in the compositions of the present invention is at least20 weight percent based on the total weight of the composition. Thepreferred amount of (A) is from 25 to 40 weight percent based on thetotal weight of the composition.

The polydiorganosiloxane (B) contains dimethylsiloxane units,diphenylsiloxane units, and methylphenylsiloxane units to provide on theaverage less than 350 siloxane units, preferably less than 100 siloxaneunits. The polydiorganosiloxane (B) is preferably endblocked with(gamma-methyacryloxyalkyl)dimethoxy- siloxy units where the alkyl ispreferably propylene or isobutylene and preferably contain on theaverage from 10 to 60 siloxane units per molecule. The most preferredcompositions contain polydiorganosiloxane (B) in which there are from 40to 60 siloxane units per molecule. The polydiorganosiloxanes can besingle polymers which provide the required average number of siloxaneunits per molecule or they can be mixtures of polymers which provide therequired average number of siloxane units per molecule. Thesepolydiorganosiloxanes of (B) can be prepared, for example, by mixinghydroxyl endblocked polydiorganosiloxane, a silane such asgamma-methyacryloxypropyltrimethoxysilane orgamma-methacryloxyisobutyltrimethoxysilane in the presence of acatalytic amount of alcoholic potassium hydroxide. It is recommendedthat polymerization inhibitor be used during this process and caninclude such compounds as phenothiazine and para-methoxyphenol. A slightexcess of the theoretical amount of silane is recommended to ensurecomplete reaction such as 10 weight percent excess. The ingredients aremixed and heated to remove the stoichiometric amount of methanol (inthis illustrated method). After the stoichiometric amount of alcohol isremoved, the resulting mixture is neutralized with acetic acid. Thepolydiorganosiloxanes of (B) have a general formula ##STR5## where B isH or methyl, Q' is a divalent alkylene radical such as propylene orisobutylene R is an alkyl radical such as methyl, ethyl or propyl, eachR" is methyl or phenyl, and n is an average value of less than 350. Theresulting polydiorganosiloxane (B) made from the process defined hereinmay contain some unreacted silane because an excess is used. Becausesuch an excess is usually in the range of less than 20 weight percent,the compositions of this invention can include the excess silane such asfrom 5 to 15 weight percent based on the weight of the product of theprocess. The polydimethylsiloxane (B) is present in an amount of atleast 20 weight percent based on the total weight of the composition.The preferred compositions of this invention contain from 20 to 30weight percent polydiorganosiloxane (B) based on the total weight of thecomposition.

The reactive diluent (C) is used to reduce the viscosity of thecomposition and to provide tougher cure films and coatings. The reactivediluent must be compatible with the rest of the ingredients of thecomposition and react during the curing processes so that they becomepart of the cured product. Reactive diluents include isobornyl acrylate,cyclohexyl acrylate, and 2-ethylhexyl acrylate. The compositions canalso contain small amounts of multifunctional acrylates, such as up to10 weight percent based on the weight of the composition. Reactivediluent (C) is preferably isobornyl acrylate. Reactive diluent (C) ispresent in the composition in an amount of at least 20 weight percentbased on the total weight of the composition. The preferred compositionof this invention contain an amount of isobornyl acrylate of from 25 to45 weight percent based on the total weight of the composition.

The photoinitiator (D) is present in an amount of from 0.5 to 10 weightpercent of a combination based on the total weight of the composition inwhich there is at least 0.25 weight percent of

C₆ H₅ --C(O)C(CH₃)₂ --OH

and at least 0.25 weight percent of ##STR6## Photoinitiator combination(D) is preferably present in an amount of from 1 to 5 weight percent inwhich 2-hydroxy-2methylphenylpropanone provides an amount of from 1.5 to3 weight percent and2-methyl-1-[4-(methylthio)phenyl]-2-morpholenopropanone provides anamount of from 1.5 to 3 weight percent. It was quite unexpected that acombination of photoinitiators together with an amine and organictitanate would provide a fast UV curing composition which also hadmoisture curing characteristics. Although some other photoinitiatorcombinations improved the rate of UV cure, the overall properties of thecompositions of the present invention were superior as a conformalcoating for printed wire boards.

Amine (E) is dimethylethanolamine or diisopropylethylamine and ispresent in an amount of from 0.1 to 2 weight percent, preferably from0.5 to 1.5 weight percent, wherein the weight percentages are based onthe total weight of the composition. These amines reduce the surfacetackiness. The preferred amine is dimethylethanolamine.

The organic titanate catalyst of (F) provides the moisture curabilityfor the composition and can be any of the conventional organic titanatessuch as tetraisopropyl titanate, tetrabutyl titanate,bis(acetylacetonate)diisopropyl titanate, andbis(ethoxyacetylacetonate)diisopropyl titanate. The amount of theorganic titanate should be sufficient enough to catalyst the compositionwhen exposed to atmospheric moisture after having been exposed toultraviolet radiation. This amount of organic titanate is from 0.05 to 1weight percent based on the total weight of the composition. Thus, anyorganic titanate which undergoes some reaction from ultravioletradiation may not be suitable if it looses its ability to cause thecuring reaction in the shadowed areas. Also the amount of organictitanate should not be so great as to influence the ultravioletradiation curing mechanism. A preferred range of organic titanate isfrom 0.05 to 1 weight percent based on the total weight of thecomposition. A preferred organic titanate is tetrabutyl titanate.

The compositions of the present invention can also contain aphotosensitizer which can influence the ultraviolet radiation curing byimproving the rate of cure and uniformity of the cure. Thesephotosensitizers include dyes, for example,4-methyl-7-(N,N-diethylamino)-coumarin. These photosensitizers arepreferably present in small amounts such as less than 0.5 weight percentbased on the total weight of the composition.

Adhesion additive can also be present in the present compositions. Oneadhesion additive which has been found to be useful is a mixture ofhexamethoxysilethylene and pentaerythritol, which is particularly usefulin the weight ratio of 40 to 60 weight percent of thehexamethoxysilethylene and 40 to 60 weight percent of the pentaerthritolbased on the weight of the adhesion additive. The hexamethoxysilethylenehas a formula (CH₃ O)₃ SiCH₂ CH₂ Si(OCH₃)₃

These adhesion additives ensure the adhesion of the conformal coating tothe printed circuit boards.

The compositions of the present invention can also contain certain otheradditives which are not detrimental to the curing characteristics thestorage stability, the cured film, and the usefulness of the cured film.Such additives include antioxidants and storage stabilizers.

Substrates which are populated with devices having geometries such thatcoating them produces areas where the composition would be in theshadows when the ultraviolet radiation is directed at the surface. Suchpopulated devices can be coated with the compositions of this inventionand can be cured in areas where the ultraviolet radiation strikes, aswell as, in the shadow area. The composition in the shadow areas willcure by merely being exposed to atmospheric moisture. Such substratesinclude populated printed circuit boards or printed wire boards.

The compositions of the present invention have two photocurablepolymers, i.e., the acrylamide functional polyorganosiloxane and themethacryloxy functional polydiorganosiloxane. This two polymercombination cures faster when exposed to ultraviolet radiation thancompositions containing only the methacryloxy functionalpolydiorganosiloxanes. The methacryloxy functional polydiorganosiloxaneis required because it provides the groups which have the ability tomoisture cure. The compositions of the present invention also cure toclear materials. This characteristic is observed when materials, whichwould otherwise be incompatible when reacted, coreact. If the twofunctional polymers did not coreact, the cured products would result inhazy or opaque materials, if they cured at all. Compositions whichcontain these two types of photocurable polyorganosiloxanes aredescribed in my copending application, entitled "Compositions Having UVCure With Moisture Shadow Cure", which is assigned to the same assigneeand was filed on even date herewith.

The compositions of the present invention have a combination ofproperties including being solventless, being low in viscosity (0.07 to0.2 Pa.s, when measured at 25° C.) and curing to flexible materialsexhibiting cold temperature flexibility. Being solventless and low inviscosity is a requirement for conformal coatings for printed circuitboards, especially when they have dual cure, UV and moisture, and whenthe resulting cured films and coatings have low temperature flexibility.The low viscosity is required so that the composition can be coated bydip coating or spray coating.

The compositions of the present invention are useful as conformalcoatings for printed circuit boards or printed wire boards, depending onthe terminology one desires to use. For the purposes of the presentinvention these terms are interchangeable. The preferred compositionscure by exposure to a dose of ultraviolet radiation in the amount of aslittle as 200 millijoules per square centimeter (in air) and stillmoisture cure in the shadow areas or dark areas, i.e. where thecomposition is not exposed to the ultraviolet radiation. Thecompositions are exposed to ultraviolet radiation to cure in air,because they cure so fast under an inert atmosphere, such as nitrogen,that the surface cures before the material below the surface even thoughit may only be one to three mils in thickness. The fast UV cure under annitrogen atmosphere might be used in applications where the coatings arevery thin, basically a surface cure. The source of the ultravioletradiation is not critical as long as cure can be obtained. It ispreferred to use a broad spectrum wavelength ultraviolet source forradiating the composition to provide cure. Many sources are availablecommercially.

The following examples are illustrative of the invention which isproperly delineated in the claims. In the following examples part orparts represent parts by weight and the viscosity measurements are at25° C. unless otherwise stated. The compositions in the followingexamples were cured in an air atmosphere using an LCU (laboratory curingunit) from UVEXS, Inc. Mountain View, Calif., which had a stainlesssteel belt on which the samples were placed for curing, and which hadbelt speeds of 3 to 14 feet per minute. This LCU had a medium pressuremercury vapor six inch lamp with a power of 200 watts per inch. Thedosage was controlled by the belt speed and the number of passes throughthe LCU. The dosages were measured by using an International LightCompany compact radiometer.

EXAMPLE 1

Compositions were prepared by blending together 37 parts of anacrylamide polydimethylsiloxane (Polymer A) of the average formula##STR7## in which y had an average value of about 90, 26.1 parts of amethacrylate polydimethylsiloxane (Polymer B) having the followingaverage formula ##STR8## where the polydiorganosiloxane was 20.1 partswherein x had an average value of 30 and 6 parts wherein x had anaverage value of 300, 32 parts of isobornyl acrylate, and 0.2 part oftetrabutyl titanate. Several compositions were prepared in which theamount of photoinitiator, 2-hydroxy-2-methylphenylpropanone (PI-1) and2-methyl-1-[4-(methylthio)phenyl]-2-morpholenopropanone (PI-2),dimethylethanolamine (Amine), and 4-methyl-7-(N,N-diethylamino)-coumarin(PS-1, photosensitizer) were varied and are shown in Table 1. Testcoupons of each composition were dip coated and then exposed toultraviolet radiation to provide a dosage of 1200 mJ/cm². Coated testcoupons were also stored in the dark under atmospheric conditions toobserve the moisture curing properties. The results were as described inTable 1. Under surface wrinkling, 4=wrinkles at or less than 5 mils;6=wrinkles at more than 5 mils; and 10=no wrinkling at 10 mils. Undersurface cure, 1=dry or very slightly tacky surface; 2=slightly tackysurface: 3=tacky surface but no smear; 4=slight surface smear; 5=severesurface smear; and 6=oily surface.

                                      TABLE 1                                     __________________________________________________________________________                      UV      UV                                                  COMPOSITION       SURFACE SURFACE                                                                             ELONGATION                                                                             MODULUS                              No.                                                                              PI-1                                                                             PI-2                                                                             AMINE                                                                              PS-1                                                                              WRINKLING                                                                             CURE  %        kPa                                  __________________________________________________________________________    1  1.85                                                                             1.85                                                                             1.20 0.015                                                                             10      3     50       2765                                 2  2.50                                                                             1.85                                                                             0.75 0.015                                                                             6       2     52       10632                                3  2.50                                                                             1.85                                                                             1.20 0.01                                                                              4       3     24       5757                                 4  1.85                                                                             1.20                                                                             1.20 0.01                                                                              6       1     74       5116                                 5  1.85                                                                             1.85                                                                             0.30 0.015                                                                             6       2     65       7157                                 6  1.85                                                                             1.20                                                                             0.30 0.01                                                                              10      1     40       7867                                 7  1.20                                                                             1.85                                                                             0.30 0.01                                                                              6       3     57       2248                                 8  1.85                                                                             1.20                                                                             0.75 0.015                                                                             6       2     52       3130                                 9  1.20                                                                             1.85                                                                             0.75 0.005                                                                             10      4     55       4992                                 10 1.85                                                                             2.50                                                                             0.30 0.01                                                                              10      2     144      4585                                 11 2.50                                                                             1.85                                                                             0.75 0.005                                                                             10      5     69       4475                                 12 1.85                                                                             1.85                                                                             0.75 0.01                                                                              6       3     43       2220                                 13 1.85                                                                             1.85                                                                             0.30 0.005                                                                             6       4     92       9735                                 14 1.85                                                                             1.85                                                                             0.75 0.01                                                                              6       2     52       4833                                 15 1.20                                                                             1.85                                                                             1.20 0.01                                                                              10      4     33       2577                                 16 1.85                                                                             2.50                                                                             0.75 0.005                                                                             10      1     126      4654                                 17 2.50                                                                             1.2                                                                              0.75 0.01                                                                              10      2     111      2351                                 18 1.85                                                                             2.50                                                                             1.20 0.01                                                                              10      1     13       2379                                 19 1.85                                                                             1.20                                                                             0.75 0.005                                                                             10      2     102      5357                                 20 1.20                                                                             1.20                                                                             0.75 0.01                                                                              6       5     102      1551                                 21 1.85                                                                             1.85                                                                             1.20 0.005                                                                             10      2     60       1606                                 22 2.50                                                                             1.85                                                                             0.30 0.01                                                                              6       2     55       3171                                 23 1.20                                                                             1.85                                                                             0.75 0.015                                                                             6       3     74       3013                                 24 2.50                                                                             2.50                                                                             0.75 0.01                                                                              10      3     52       6322                                 25 1.85                                                                             2.50                                                                             0.75 0.015                                                                             6       4     85       3647                                 26 1.20                                                                             2.50                                                                             0.75 0.01                                                                              6       2     72       2392                                 27 1.85                                                                             1.85                                                                             0.75 0.01                                                                              6       2     21       4240                                 __________________________________________________________________________

Compositions 4, 6, 10, 11, 16, 20, and 24 were specifically tested formoisture cure. These were all adequately cured in six days when storedin the dark under atmospheric conditions. All 27 of the abovecompositions are of this invention. These compositions show variousresults, however, all were acceptable conformal coating compositionseven considering the differences. Many of the compositions which showedwrinkling at five mils thickness are expected to be useful as lower filmthicknesses.

EXAMPLE 2

A composition was prepared by mixing 37 parts of Polymer A, 26.1 partsof Polymer B 32 parts of isobornyl acrylate, 1.8 parts of PI-1, 2.2parts of PI-2, 0.3 part of Amine, 0.01 part of PS-1, 0.2 part of anadhesion additive which was a blend of 45 weight percent ofhexaethoxysilethylene and 55 weight percent of pentaerythritol, and lessthan 500 ppm of para-methoxyphenol and phenothiazine as stabilizers. Theresulting mixture has a viscosity of 0.08 Pa.s and had a shelf life ofgreater than three months stored in a sealed container in the dark. Testcoupons were prepared as described in Example 1 and cured by exposure toUV radiation at a dosage of 1500 mJ/cm². The coating was about five milsin thickness. The composition cured to an excellent conformal coatingwithout forming a wrinkled surface It adhered to the coupon surface andwhen used on a populated board it cured both by the UV exposure and bymoisture cure in the dark or shadowed areas.

That which is claimed is:
 1. In a method comprising coating a substratepopulated with devices having geometry such that there are areas whichare in shadows when light is directed onto the substrate, exposing thecoated substrate with ultraviolet radiation and thereafter allowing theexposed substrate to further cure by the reaction of moisture at roomtemperature, where the improvement comprises using a coating of acomposition comprising(A) at least 20 weight percent based on the totalweight of the composition of an acrylamide polydimethylsiloxane havingacrylamide end groups bonded to silicon atoms through Si--C bonds and onthe average at least 20 to less than 300 dimethylsiloxane units permolecule, (B) at least 20 weight percent based on the total weight ofthe composition of a polydiorganosiloxane having endgroups containingboth alkoxy groups and methacrylate groups where there are two alkoxygroups per endgroup bonded to silicon through an Si--O bond and onemethacrylate group bonded to silicon through an Si--C bond and having onthe average less than 350 diorganosiloxane units per molecule, saiddiorganosiloxane units being selected from dimethylsiloxane units,diphenylsiloxane units, and methylphenylsiloxane units wherein at least50 percent of the organic groups of the diorganosiloxane units aremethyl, (C) at least 20 weight percent based on the total weight of thecomposition of a reactive diluent selected from the group consisting ofisobornyl acrylate, cyclohexyl acrylate, and 2-ethylhexyl acrylate, (D)from 0.5 to 10 weight percent based on the total weight of thecomposition of a photoinitiator combination in which there is at least0.25 weight percent of based on the total weight of the composition

    C.sub.6 H.sub.5 --C(O)C(CH.sub.3).sub.2 --OH

and at least 0.25 weight percent of based on the total weight of thecomposition ##STR9## (E) from 0.1 to 2 weight percent based on the totalweight of the composition of an amine selected from the group consistingof dimethylethanolamine and diisopropylethylamine, and (F) from 0.05 to1 weight percent based on the total weight of the composition of anorganic titanate, where the composition has a viscosity at 25 degrees C.in the range of 0.07 to 0.2 Pa.s.
 2. The coated substrate obtained fromthe method of claim
 1. 3. The coated substrate obtained from the methodof claim 1 in which the substrate is a printed circuit board.